1. Field of the Invention
This invention relates to an image forming apparatus, such as a laser printing apparatus, for forming an image on an image bearing member and, more particularly, to a detachably mounted toner unit including a toner supply unit for supplying toner to a developing device and a toner recovery housing unit for accommodating toner removed from an image bearing member by a cleaning device.
2. Description of the Related Art
Generally, in conventional image forming apparatus, a surface of an image bearing member, such as a photosensitive drum, is charged by a main charger of the apparatus. The entire surface of the drum is uniformly charged by the main charger. Then, a latent image is formed on the image bearing member. A developing device then develops the latent image using a developing agent (toner) to make the latent image visible. Subsequently, the developed image is transferred to a medium, such as a paper sheet, by an image transfer device. Most of the toner of the developed image on the image bearing member is transferred to the paper sheet by the image transfer device during the image transfer process. However, some portion of the toner of the developed image remains on the image bearing member after the transfer of the developed image to the paper sheet by the image transfer device. Thus, the residual toner particles on the image bearing member are typically removed from the image bearing member by a cleaning device of the apparatus.
In this conventional image forming apparatus, the toner is consumed during every image formation so that it is necessary to replenish the supply of toner for use by the developing device. Also, it is necessary to accommodate any recovered toner removed from the image bearing member by the cleaning device. Thus, the image forming apparatus has a toner supply unit for supplying the toner to the developing device and a toner recovery housing unit for housing the recovered toner removed from the image beating member by the cleaning device.
In recent image forming apparatus designs, several elements of the apparatus have been contained in one casing, so as to form a replaceable unit (herein referred to as a "processing unit"). For example, U.S. Pat. No. 3,985,436 discloses an image forming apparatus wherein the image bearing member, the developing device having the toner supply unit and the cleaning device having the toner recovery housing unit are contained in a single unit. This processing unit may be detached from the apparatus and replaced with a new processing unit upon expiration of its service life. Thus, all the devices contained within the single processing unit are exchanged simultaneously.
However, in this processing unit, the capacity for storing the developing toner and the recovered toner are limited because of the practical size of the processing unit. Therefore, the exchange cycle for this processing unit is short because of its capacity limitations, and the processing unit must be exchanged early and well before the expiration of the service life of the image bearing member. Thus, the operating cost for operating the known image forming apparatus is high.
In another embodiment, the toner supply unit and the toner recovery housing unit are separate from one another and are each adapted to be detachable. The toner supply unit may be detached from the developing device and the toner housing recovery unit can be detached from the cleaning device, and thus separately replaced as required. However, in this structure, a user must exchange either the toner supply unit or the toner recovery housing unit each time the toner supply is empty or the toner recovery housing unit is full. Thus, it is very troublesome for the user to have to exchange the toner supply unit or the toner recovery housing unit.